Follicle stimulating hormone (FSH) stimulates the formation of colonies by 10 day old rat testis cells in primary culture. The FSH responsive cells have been identified as Sertoli cells based on their ultrastructure and the presence of nucleolar satellite karysomes, structures unique to the Sertoli cell. In addition to FSH, dibutyryl cyclic AMP will stimulate colony formation (peak stimulation at 1mM). Interestingly, the stimulation at peak dibutyrl CAMP levels is significantly greater than that seen with peak FSH doses. Theophylin, caffeine, or methyisobutyl xanthine do not increase the responses to FSH or dibutyryl CAMP suggesting that phosphodiesterases may be absent. The increased response to dibutyryl CAMP is noted with preparations from the whole testis and with isolated Sertoli cells. Formation of colonies requires pipetting of the cell suspension suggesting the presence of a Sertoli cell surface recognition factor. In addition, treatment with 5 microns NaIO4 inhibits colony formation and the FSH stimulation. Addition of NaIO4 after the cells are plated has no effect. Ghicoseamine (5mM) significantly stimulated colony formation in control but not in FSH-treated cultures. These data suggest that FSH may be stimulating changes in cell surface proteins.